IOWA ACADEMY OF SCIENCE 
43 
species. Clearly then the idea of disease implies danger of premature death, 
and probably this is as near as we shall get to a satisfactory definition.” 
“If we agree that a living plant in a state of health is not a fixed and un- 
altering thing, hut is ever varying and undergoing changes as its life works 
out its labyrinthine course through the vicissitudes of the ever-varying en- 
vironment, then we cannot escape the conviction that a diseased plant, so long 
as it lives,' is also varying in response to the environipent. The principal 
difference between the two cases is, that whereas the normal healthy plant 
varies more or less regularly and rhythmically about a mean, the diseased one 
is tending to vary too sudenly or too far in some particular direction from the 
mean; the healthy plant may, for our present purposes, be roughly likened 
to a properly balanced top spinning regularly and well, whereas the diseased 
one is lurching here, or wobbling there, to the great danger of its stability. 
For we must recognize at the outset that disease is but variation in directions 
dangerous to the life of the plant. Health consists in variation also, but not 
in such dangerous grooves. That the passage from health to disease is gradual 
and ill-defined in many cases will readily be seen.” 
“Now take the case where the roots are maintaining their maximum func- 
tional activity, but the leaves — owing to want of light, too much moisture or 
too low a temperature of the air — are functionally depressed. Here we get a 
state of oversaturation with water set up, the tissues are turgid to bursting 
point, what supplies do traverse the sieve-tubes, cortex, etc., do so slowly and 
are excessively diluted, and the cambium again forms less wood, but the lumina 
of the vessels are larger and the lignification less complete. Growth in length 
is excessive, but more leaves are formed, though they are apt to be abnormally 
thin and may be small. Little or no reserves are stored anywhere, and the 
watery tissues contain dangerously diffusible substances which may render 
them an easy prey to parasitic fungi. Here again, however, if the disturbance 
of equilibrium has not gone too far, and if the season permits, the new leaves 
may come into full activity and the situation be saved by transpiration and 
assimilation gradually increasing and restoring the equilibrium. But, as be- 
fore, the plant has suffered, and shows the effect in its weak shoots, retarded 
fiowering, and other ways.” 
“Such plight as is here described may actually be attained in greenhouses 
where over-watering is the fault, and even in the open it is not uncommon in 
rainy summers, or in plantations where dominant trees get the upper hand 
and partially shade more slowly growing species, or in fields where rank grass 
is allowed to overwhelm crops of lower stature.” 
Plants like animals are sensitive, and the maintenance of health is dependent 
on its sensitiveness. The healthy plant therefore manifests this in the greatest 
degree. Health, therefore, is a condition in which the reaction between the 
organism and the surroundings are perfect. The condition, as J. Reynolds 
Green^®^ says in which the relationship to each of these factors (light, temperor 
ture and moisture) is satisfactory is generally spoken of as one of tone, and 
the influence which each exerts when it affects the plant uniformly is spoken 
of as a tonic influence.” 
lOaAn Introduction to Vegetable Physiology, 358, 
